German Patent No. 197 13 967, for example, describes a system for measuring distances in the surrounding area of motor vehicles having a FMCW microwave radar sensor (FMCW=Frequency Modulated Continuous Wave). This radar sensor has, as essential elements, a transmit oscillator, a mixer, and an antenna system for transmitting and receiving radar beams reflected by an object. The transmitted signal is frequency modulated with a predefined ramp function, for example, so that a frequency of the transmitted signal modified in the meantime by the modulation ramp is present due to the propagation time of the received reflected signal, and this frequency difference is a measure for the distance measurement.
The linearity of the above-mentioned ramp function, i.e., of the transmitted frequency ramp, is of decisive importance for the accuracy of measurement, the resolution, and the sensitivity of the FMCW microwave radar. In order to ensure that a linear frequency ramp is produced for modulation, the known device also has a reference oscillator whose output signal is mixed with the transmitted signal directly in the radar sensor simultaneously with the actual distance measurement. A reference quantity can be obtained by demodulating this signal, the reference quantity containing any non-linearities in the transmitted branch of the radar sensor, which can thus be taken into account accordingly in the analysis.
In conventional microwave radar systems, frequency regulation is also used (FLL or PLL control circuits) to achieve sufficiently good linearities of the frequency ramp; this regulation is implemented using appropriate additional, usually expensive, circuit resources. Such a frequency regulation and the respective hardware circuit components must be continuously monitored in operation for proper functioning, since a faulty response in the frequency regulating circuits results in modulation with insufficiently linear frequency ramps.
Although using the known devices it is possible to perform real time frequency regulation to the desired modulation frequency with correction of non-linearities, this involves relatively high circuit costs.
The present invention is based on the above-mentioned method of detecting and correcting non-linearities in a microwave radar system in which a transmitted signal, frequency modulated according to a predefined function, is generated using a transmit oscillator, and by mixing the transmitted signal with a received signal reflected by an object, a distance of the microwave radar system from the object is determined. According to the present invention, in predefined time windows a predefined constant test control voltage or a sequential series of such voltages is sent to the transmit oscillator instead of a control voltage effecting frequency modulation, provided in normal measuring cycles.
The reference signal determined for the respective constant test control voltage which should correspond to a frequency value of the transmit oscillator can now be used for correcting the characteristic curve for frequency modulation and thus for compensating for non-linearities in a simple manner. Thus, in principle the present invention can be summarized as measuring the voltage/frequency characteristic of the transmit oscillator operating in the mm wavelength range using a test function.
Test control of the oscillator is achieved using the constant control voltage which is applied to the transmit oscillator (VCO) for the predefined time window. The test voltage is converted into a transmitted frequency in the transmit oscillator and the transmitted frequency is mixed with the frequency of a reference oscillator (DRO), for example, with its sixth harmonic. The mixed frequency is then proportional to the emitted oscillator frequency, the frequency of the reference oscillator being selected so that the mixed frequency is in the range of  less than 1 GHz.
For the usual type of ramp as a modulation function for the transmit oscillator, a ramp function which is appropriately pre-distorted in the voltage range is then generated from the measurement results; this ramp function takes into account and compensates for the non-linearities of the voltage/frequency characteristic of the transmit oscillator. These non-linearities may be caused, for example, by a non-linear frequency ramp in normal operation due to a fault of the ramp generator or of a regulating circuit (PLL or FLL), or also by a malfunction of a reference oscillator (DRO) or a defective U/f conversion by the transmit oscillator (GUNN oscillator).
According to a preferred embodiment, the method according to the present invention is performed using a stepped test ramp, whose individual steps form time window xcex94t each with test control voltages of different magnitudes and whose steps each have a predefined deviation, which may also be constant. The actual frequency value of the transmit oscillator is then determined at each step.
This linearity test is performed cyclically during the operation of the microwave radar system, for example, once a second. Advantageously, after a predefined number of measuring cycles, preferably in every 10th measuring cycle and once at the time of the initialization of the microwave radar, the stepped test ramp is advantageously applied to the transmit oscillator and is subsequently evaluated so that the difference of the deviations of the frequency values with respect to the deviations of the test ramp is determined from the measured frequency values. The absolute values of the deviation differences of adjacent steps are added up and the sum is compared to an error-threshold.
In order to keep the memory requirements necessary for the above-mentioned calculation low, a linearity indicator xcex4, determined by successive summations of the deviation differences, is formed. For this purpose, the transmit oscillator is stepped up in constant steps from the lowest possible frequency over the entire control range, whereby ideally a stepped curve with a constant deviation is obtained for the intermediary frequency. In the following step, the absolute value of the deviation difference |xcex94Hub| from step n to step nxe2x88x921 is formed for a total number of k steps and subsequently added up over all deviation differences. The following equation is thus obtained for linearity indicator xcex4:                     δ        =                              ∑                          n              =              1                                      k              -              1                                ⁢                      "LeftBracketingBar"                                          Δ                ⁢                                  xe2x80x83                                ⁢                                  Hub                                      n                    +                    1                                                              -                              Δ                ⁢                                  xe2x80x83                                ⁢                                  Hub                  n                                                      "RightBracketingBar"                                              (        1        )            
where
Hubn=fstufe(n)xe2x88x92fstufe(nxe2x88x921)xe2x80x83xe2x80x83(2)
The frequency of the reference signal obtained by mixing can be subdivided according to an advantageous embodiment using a frequency divider until it can be measured during the predefined time window with sufficient accuracy. A direct correspondence is thus obtained between the value of the test voltage applied and the oscillator frequency.
The present invention advantageously makes it possible to update the voltage/frequency characteristic of the module during the above-mentioned time window in order to generate the function for frequency modulation taking into consideration the reference signal, and to use the updated voltage/frequency characteristic for frequency modulation of the transmit oscillator outside the time window during operation of the microwave radar system. Thus the entire voltage/frequency characteristic can be determined or updated in a simple manner by varying the test control voltage in consecutive time windows if it has changed, for example, due to temperature influences. Subsequently the characteristic thus determined can be taken into account in generating a modulation ramp for the variation of the control voltage in normal operation, so that a linear frequency ramp is transmitted by the microwave radar as a result.
Overall, the method according to the present invention allows for a cost-effective design of a microwave radar system having the required frequency regulating components, since the voltage/frequency characteristic can be updated, i.e. corrected, via control commands integrated in a software program for ramp generation in order to modulate frequency without any additional circuit components. Furthermore, the proposed method is relatively insensitive to fluctuations in the values of the control electronics components in addition to being easily adjusted to new operating conditions.